The present invention relates to a heat exchanger having a heat pipe formed of a space between an outer tube and an inner tube inserted in the outer tube and, more particularly, to a heat exchanger for exchanging heat between a hot fluid flowing around the outer circumference of the outer tube and a cold fluid flowing in the inner tube.
In a known heat exchanger using a heat pipe, this pipe is interposed between a hot fluid passage and a cold fluid passage. The heat exchanger of this type is advantageous in that the heat pipe can effect the heat exchange highly efficiently because it transfers the heat as the latent heat of a working fluid confined therein, and in that the heat exchange is not troubled in the least even if the hot fluid passage and the cold fluid passage are disposed apart from each other because the heat pipe can transfer the heat over a long distance through evaporation and flows of the working fluid. Despite of these advantages, however, the heat exchanger must have its heat pipe exposed at its one end to the hot fluid and its other to the cold fluid. In order to retain a wide heat transfer area, therefore, it is necessary to enlarge the heat pipe in length and diameter. This necessity raises a disadvantage that the heat exchanger is large-sized in its entirety.
In order to avoid this disadvantage, there has been proposed a heat pile which has a dual-tube structure. In this structure, an inner tube is inserted in an outer tube to form a sealed space between the inner circumference of the outer tube and the outer circumference of the inner tube. This space is evacuated and then confined with a condensible fluid such as water as its working fluid.
A radiator using the dual-tube heat pipe is disclosed in the specification of Japanese Patent Laid-Open No. 56 - 27891 or on page 116 of "Heat Pipe and its Applications" (published by OHM K.K.), for example. This radiator is constructed such that the inner tube for a hot fluid is so inserted with a lower eccentricity in the outer tube arranged generally in a horizontal position and formed with fins on its outer circumference that it is immersed in the working fluid while forming the space between those outer and inner tubes into the heat pipe. As a result, the outer circumference of the inner tube acting as an evaporator is sufficiently fed with the working liquid by the action of a wick because the inner tube is partially immersed in the working fluid. This fluid is evaporated by the heat transferred from the hot fluid flowing in the inner tube, and its resultant vapor comes into contact with the inner circumference of the outer tube so that its heat is robbed by the external fluid at a lower temperature. In other words, the vapor releases its heat to condense into the working liquid, which then drops on the inner circumference of the outer tube to form a liquid sump. The working liquid is fed again for reuse to the outer circumference of the inner tube by the wick action.
In the radiator of the dual-tube heat pipe type, in which the hot fluid flows in the inner tube whereas the cold fluid flows outside the outer tube, the working fluid will stagnate on the bottom of the outer tube. This stagnation makes it necessary to offset the inner tube for the hot fluid downward with respect to the outer tube so that the inner tube may be partially immersed in the working fluid. In case, on the contrary, the flows in the inner tube whereas the hot fluid flows outside of the outer tube, the working fluid is heated and evaporated, even if stagnant on the bottom of the outer tube, by the heat of the hot fluid transferred through the outer tube. The resultant vapor of the working fluid comes into contact with the outer circumference of the inner tube so that it is cooled and condensed. In connection with the heat transfer to and from the working fluid, there arises no trouble even if the outer tube and the inner tube are concentrically arranged. Therefore, the heat exchanger using the dual-tube heat pipe for exchanging the heat between the cold fluid flowing in the inner tube and the hot fluid flowing outside of the outer tube is constructed such that the inner tube is concentrically inserted in the outer tube, which is lined with a wick, as disclosed in the specification of Japanese Patent Laid-Open No. 61 - 235688, for example.
In case the heat is to be transferred from the hot fluid outside of the outer tube to the cold fluid in the inner tube by the dual-tube heat pipe having its outer and inner tubes arranged in a concentric relation, the heat pipe is arranged with a horizontal axis to cause the overall inner circumference of the outer tube to act as the evaporator. Therefore, the wick is generally extended over the inner circumference of the outer tube. In case, however, the heat exchange is to be accomplished between molten hot sodium and water, for example, the heat flow is too high for the working fluid to extend all over the inner circumference of the outer tube, even if it is scooped by the capillary action of the wick, from the liquid sump formed on the bottom of the outer tube so that it is evaporated midway. As a result, the working fluid is insufficient at the top portion of the inner circumference of the outer tube to invite the so-called "dry out". Thus, the heat pipe of the dual-tube structure having its outer and inner tubes concentrically arranged is accompanied by a problem that the substantial area of the evaporator is restricted to drop the heat exchanging efficiency in case of the heat exchange between the hot fluid flowing outside of the outer tube and the cold fluid flowing in the inner tube.